1. Field of the Invention
The present invention relate's to a magnetron for use in a microwave heating instrument such as a microwave oven, and more particularly to an improved cathode support structure for a magnetron.
2. Description of the Prior Art
An example of a conventional cathode support structure for a magnetron is disclosed in Japanese Patent Application Laid-open No. Sho 62-76241, in the above Application, in order to enhance a productivity and a reliability, a metallic layer is deposited on an upper surface of a ceramic stem to which cathode support rods are to be fixed. An upper surface of the ceramic stem to which a metal sleeve is to be fixed is adapted to be lower than the metallic layer to be fixed to the cathode support rods, and is deposited with a metallic layer.
With the above-constructed cathode structure, although it is possible to improve a productivity and a reliability of the magnetron, long lengths of the cathode support rods cause a production cost of the magnetron to be increased due to its expensive molybdenum. Also, a vibration of the cathode structure is hard to be reduced because of the long cathode support rods.
Another conventional cathode support structure for resolving the above problems is illustrated in FIG. 1. As shown in the drawing, a coiled filament cathode 1 made of thorium and molybdenum is fixed at its both ends to a pair of end caps 2 and 3, which are made of molybdenum. A cathode support rod 5 made of molybdenum is passed through the coiled filament cathode 1 without being in contact with the filament cathode 1 and is fixed at its upper end to the upper end cap 2.
A cathode support rod 4 made of the same material as that of the cathode support rod 5 is fixed to the lower end cap 3. The other ends of the cathode support rods 4 and 5 are passed through holes formed fin an insulating ceramic stem 7 constituting a part of the cathode support structure and electrically connected to terminal strips 8 and 9 by using a brazing.
The lower ends of the cathode support rods 4 and 5 are brazed to the terminal strips 8 and 9 which are fused to a metallic layer 7B deposited on a bottom surface of the ceramic stem 7. A circumference 7D around a recess 7C formed in a top of the ceramic stem 7 is also deposited with a metallic layer. A metal sleeve 6 is hermetically fused to the circumference 7D. Pole pieces and a cylindrical anode (not shown) are hermetically fused to a flange portion 6a of the metal sleeve 6.
However, in the above-mentioned cathode support structure for a magnetron, since a length "l" of the cathode support rod 5 spanning between its lower fused end and the upper end cap 2 is fairly long, a production cost of the cathode support rods and thus the magnetron is increased because the cathode support rods are made of expensive molybdenum material.
Also, when the magnetron i s subjected to an external impact, the fragile filament cathode is easily damaged because an amplitude of vibration generating from the upper end of the cathode support rod, and thus from, the upper end cap 2, is high.
Furthermore, when the pair of cathode support rods 4 and 5 inserted through the ceramic stem 7 are assembled with the terminal strips 8 and 9, respectively, the assembling operation is considerably complicated since the terminal strips are small and the cathode support rods are assembled within the recess formed in the ceramic stem 7.
In addition, in case of the above-mentioned cathode support structure for a magnetron, when an axis of the filament cathode 1 made of thorium-tungsten alloy does not coincide with an axis of the cylindrical anode (not shown), an increased current loss occurs due to leakage into an region adjacent to the filament cathode 1 and anode vanes, which does not contribute to oscillation. This causes oscillation efficiency deteriorate.
For this reason, since the distribution of thermions emitted from the filament cathode 1 is not uniform and thus the thermions concentrate in the region adjacent to the anode vanes, uneven electrical energy is applied to a plurality of resonators defined by the anode vanes, thereby causing microwave output to be decreased.
Also, temperature of the filament cathode 1 adjacent to the region in which the thermions concentrate is rapidly increased, so that the thorium inside the filament cathode 1 which is necessary to emit the thermions is rapidly vaporized. Hence, thermion emitting ability of the filament cathode 1 is rapidly decreased, shortening the service life of the magnetron.
Furthermore, since dimension error of the cathode support rod 5 may occur when the cathode support rod 5 is bent at its two positions, that is, at portions of circle A and circle B in FIG. 1, the cathode support rod 5 is difficult to be precisely located in the axis of the filament cathode 1 after the cathode structure is assembled. Accordingly, the cathode structure deviates from an axis of the cylindrical anode, so that an efficiency and an output as well as a service life of the magnetron is decreased.
In order to overcome the above problems by adjusting the position of the cathode support rod 5 after assembly of the cathode structure, a production cost is increased because of the additional adjusting operation.